As a high-pressure polymorph of olivine, Wadsleyite (beta-Mg2SiO4) is expected to be a dominant mineral in the transition zone from 410 km to 520 km depth in the mantle. Previous studies show wadsleyite can incorporate variable amounts of water up to greater than 3 wt% of water (Kohlstedt et al. 1996; Inoue et al. 1995; Smyth et al. 1987, 1994). Small amounts of water could strongly influence a number of physical properties of wadsleyite. The effects of water on the elasticity of wadsleyite are poorly known. We measured the single-crystal elastic constants of wadsleyites with 0.37 wt%, 0.84 wt% and 1.66 wt% H2O at ambient conditions by Brillouin spectroscopy. By computing the aggregate elastic properties, we find that the bulk (K_S0) and shear modulus (G_0) of hydrous wadsleyite decrease linearly with water content according to the following relations: K_S0=170.3-12.4X_W; G=111.8-7.9X_W; where X_W is the water H2O weight percent. Compared with anhydrous wadsleyite, 1 wt% of water will lead to 7.3% decrease in bulk modulus, 7.1% decrease in shear modulus. Water has a greater or comparable effect on the elastic moduli of wadsleyite as that of olivine or ringwoodite. To quantify the effect of pressure on the elasticity of hydrous wadsleyite, we also carried out Brillouin measurements for samples containing 0.84 wt% H2O to 12 GPa. Pressure derivatives of the bulk and shear moduli of wadsleyite with 0.84 wt% H2O are 4.2(1) and 1.4(1) respectively. These values are not significantly different from the corresponding values of anhydrous wadsleyite (e.g. Zha et al., 1997). Thus, the presences of 0.84 wt% H2O has no detectable effect on the pressure derivatives of the bulk and shear moduli.

Using this data, we first examine the effect of H2O on bulk sound velocities under transition zone conditions because anelastic effects can be neglected in this case. At 410 km depth (~13.8 GPa, along a 1400oC adiabat), the bulk sound velocity of wadsleyite with 1 wt% H2O is 3.1% lower than for dry wadsleyite. Comparison of the seismic velocity jump across the 410-km discontinuity with the measured velocity contrast between wadsleyite and olivine provides a means to estimate the olivine abundance at 410-km depth. For mantle wadsleyite with 0.1-0.2 wt% H2O (Huang et al., 2005) and using experimentally determined olivine-wadsleyite H2O partition coefficients, the olivine abundance is found to be 40%, much lower than a pyrolite model. In order for a pyrolite composition to satisfy the seismic data, 1.2 wt.% H2O is needed in wadsleyite - a value greater than its maximum solubility under these conditions. The anomalously steep seismic gradient in the transition zone has been another feature of the region that has long defied explanation. We show that the seismic gradient can be matched if there is a gradient in H2O concentration across the transition zone such that the H2O content drops, for example, from 0.3 wt% at 410 km to 0.1 wt% at 520 km dpeth. For compressional and shear wave velocities, 0.1 wt% H2O in wadsleyite would lead to 0.3% and 0.4% reductions in VP and Vs, respectively, neglecting any anelasticity. If the water content of wadsleyite was instead 1.0 wt. %, then the corresponding velocity reductions would be 3.3% and 3.6%.